The present invention relates, in general, to a treatment method for reusing fluorine-treated residue and, more particularly, to a method for re-treating with rare-earth compounds residue which is generated in the course of removing dissolved fluorine ions from waste water, so as to reuse the residue.
In the natural world, there exists a trace amount of fluorine by nature, for instance, about 1.2 to 1.4 ppm in the marine water. This trace amount of fluorine does not cause any problems in view of ecological environment. Now, the use of fluorine or fluorine-containing compounds, however, is extensively increased as various industries, such as electromagnetic, metallic, chemical or ceramic industries, have been rapidly developed. For example, fluorine or fluorine-containing compounds are used for manufacturing semiconductors, washing the interior of a cathode ray tube, manufacturing aluminum, etc.
Waste water released from a plant pertaining to the industries contains fluorine which is in a free radical state or combined state with other elements or compounds, and the concentration of fluorine in the waste water often comes to several hundreds ppm, which is very dangerous in view of ecology, affecting human bodies, animals or plants detrimentally. Accordingly, it is required to lower the concentration of the fluorides in the waste water.
In an effort to lower the concentration of fluorine in the waste water, there has been proposed such a method for treating the fluorine-containing waste water that at least one species selected from a calcium compound, e.g. Ca(OH).sub.2, an aluminum compound, e.g. Al.sub.2 (SO.sub.4).sub.3 and a phosphoric acid compound is added to the waste water, so as to insolubilize the fluorine ions, and the resulting supernatant and sludge is separated from each other.
However, the above conventional method is problematic in that a good deal of chemical reagents are needed, generating a great amount of sludge. Even though this generated sludge is buried, fluorides are eluted therefrom, so that a plurality of alkaline chemical reagents necessary to neutralize the fluorides are demanded. What is still worse, this conventional method is not so effective to the removal of the fluorine.
In order to solve the problems, rare earth compounds have recently been used as fluorine-removing agents, in lieu of calcium oxide (Ca(OH).sub.2) or aluminum sulfate (Al.sub.2 (SO.sub.4).sub.3). Rare earth elements along with fluorine form rare-earth fluorides, very stable compounds. Since these rare-earth fluorides formed are very insoluble, there is brought about an excellent effect on removing the fluorine dissolved in the waste water. In addition, the use of rare earth compounds is characterized by the fact that only a small amount of residue is generated after the treatment by virtue of the insolubility of the rare-earth fluorides.
Rare earth ore is grouped into bastnasite, monazite and xenotime. Bastnasite consists of a type of LnCO.sub.3 F wherein Ln is a general term for representing rare earth elements, such as La, Ce, Pr and the like, whereas monazite and xenotime each consists of a type of LnPO.sub.4.
Taking account into economy, the use of monazite or xenotime is improper due to its content of high-priced elements. For instance, thorium (Th) is contained in amounts ranging from 5 to 10% by weight in monazite and other high-priced rare-earth elements are contained in xenotime. In addition, such ores have elements unsuitable to remove fluorine ions, because the fluorides thereof have large solubilities, relative to that of the light rare earth elements. Therefore, in advance of using these ores, there must be carried out such a treatment that the unsuitable elements are removed therefrom, which is one factor of cost increase.
It is very difficult to reduce the treatment cost to a certain level. For example, even though the amount of the rare earth element could be reduced considerably as a result of intensive studies, it is impossible to cut down the amount thereof unlimitedly since it must be basically used in an amount of not less than equivalent weight and the amount of the removed fluorides is proportional to its amount used. The cost of manufacturing articles is raised according to the use of the high-priced rare earth elements for removing the fluorine contained in waste water and to the augmentative amount thereof. Accordingly, it is economically unadvantageous unless the reuse of the rare earth elements is accompanied by re-treating the residue which is generated in the course of removing fluorine from waste water.
Japanese Patent Laid-Open Publication No. Sho. 61-192385 (hereinafter, referred to as "the prior art") discloses that the rare earth elements can be reused by contacting an insoluble rare earth compound with waste water to absorb fluorine or fluorine-containing compounds, desorbing the absorbed fluorine with an alkaline solution and recovering rare earth elements in the forms of insoluble rare earth compounds and alkaline fluorides. The prior art is a kind of a desorption method in which the rare earth fluorides are treated with a Ca(OH).sub.2 solution or a NaOH solution, to give alkaline fluorides which are subsequently added with Ca(OH).sub.2, to precipitate calcium fluorides and to reuse the alkaline solution.
However, the prior art utilizing the alkaline solution has many difficulties. First, the dependance of alkaline solution on ion exchange reaction could cause poor reaction efficiency. In addition, after the desorption, pH is required to be lowered by neutralizing the eluting solution. Further, when the generated alkaline fluorides are treated with Ca(OH).sub.2, a great deal of CaF.sub.2 residues are formed, so that the fluorine of above a certain concentration (CaF.sub.2 solubility 8 mg/e) always remains in the solution. Furthermore, as seen from the case that the desorbing solution has to be added with the precipitating agent, a great amount of treating chemicals are needed. Accordingly, the prior art is disadvantageous in that its processes are intricate and uneconomical.
In particular, the prior art says, in detail, only about recovering the alkaline solution for reuse, but does not disclose qualities according to the recovery of rare-earth compounds, and related technical contents, neither.
By contrast with the re-treatment methods utilizing soluble rare earth compounds to remove fluorines from the waste water, re-treatment methods utilizing soluble rare earth compounds still have not been established.